Gun barrels have been made in substantially the same way since the early 1900's, with only minor improvements in processes and materials since then. The process is basically to mount a large cylindrical steel casting or forging for rotation about its axis and machine the outside to a tapered cylindrical barrel blank. The blank is then mounted in a gun drill and rotated about its axis against a drill to bore axially through the barrel blank. Finally, a broaching operation cuts shallow helical grooves to form rifling between the grooves.
Mostly through trial and error, refinements have been made to manufacturing techniques for making gun barrels to correct for inaccuracies that were noted under certain conditions of use. For example, rapid or extensive firing of the gun heats the gun barrel, and it was found that the uniformity of the barrel thickness around the barrel is important to prevent unequal thermal expansion that can distort the barrel into a curved or even wavy shape and ruin the accuracy of the gun. To minimize this type of distortion, the barrels are turned as accurately as possible after the bore as been bored, and high accuracy guns are provided with thick walled barrels to minimize the effects of whatever variations in barrel wall thickness remain.
Differential thermal expansion is also believed to be responsible for non-uniform twisting of the barrel as it heats during use, caused by the non-uniform thickness of the barrel wall due to the rifling in the bore. The slightly corkscrew shape of the barrel is also detrimental to accuracy of the gun.
The high temperature of the barrel is a consequence of high rate of fire and is considered to be inevitable. At present, the only known techniques to prevent high barrel temperature involve various types of active cooling, including the use of water jackets around the barrel. Little effort has been made to study the source of heat, which is primarily conduction from the burning propellant in the breech and the barrel, and also friction between the projectile and the bore. Reduction of this heat flux into the barrel would retard the rise in temperature of the barrel during use and alleviate some of the deleterious effects of the high temperature on barrel performance.
Conventional steel alloys used in gun barrels, including rifles, side arms, and shotguns as well as barrels for large naval and ground artillery and high rate-of-fire weapons such as machine guns and cannons are heat treatable to increase their strength. However, the trade-off for attaining high strength by heat treatment in steel alloys is an increase in brittleness. Put another way, the ability of the steel alloy to yield without rupturing when its yield strength is exceeded, a property known as toughness, is lost or reduced when the steel is heat treated to achieve high strength. High strength brittle material in a gun barrel is dangerous because overpressure caused by a plugged barrel or excessive powder loads, or weakness in the barrel caused by damage, fatigue, corrosion, or other such factors could cause the barrel to burst catastrophically instead of just bulge. Since the bursting usually occurs at the breach, near the shooter's face, the potential for serious injury, blinding, or death is high. Accordingly, it is the normal practice, although unfortunately not universal, for gun manufacturers to sacrifice potential strength and hardness of their barrel materials for toughness by not heat treating to maximum strength, usually less than 32 KSI. As a result, the barrel wall thickness must be made commensurably thicker and the soft condition of the barrel material is susceptible to rapid erosion from passage of the projectiles.
A goal in designing modern military weapons is to attain higher muzzle velocity for the projectile to attain longer range, flatter trajectory, higher impact energies and greater accuracy. One conventional technique for attaining higher muzzle velocity is to increase the barrel length to give a longer time during which the propellant gas pressure can act on and accelerate the projectile. Apart from cost, the primary limitation on barrel length is weight. The increased moment of inertia of a long barrel increases the load on the training mechanisms used to point the barrel, especially when tracking a moving target or shifting between targets in a rapidly evolving battlefield situation. Moreover, the vibration and resonant conditions are compounded in a long barrel.
Another technique for increasing the muzzle velocity is to increase the propellant energy. The limitations of this technique are the burst strength of the barrel, primarily in the breech area since the pressure spike of the reacting propellant occurs primarily while the projectile is near the breech. To flatten that pressure spike, the propellant may be adjusted to react more slowly and provide a more steady pressure against the projectile. However, the pressure pulse created by the muzzle blast from the propellant when the projectile exits the barrel must be controlled to prevent injury to personnel or equipment in the vicinity. Barrel materials that could withstand an extreme pressure pulse from a high energy propellant would enable a gun to greatly increase the muzzle velocity without creating a muzzle blast that exceeded the established safety limits.
Steel is a dense material, and gun barrels made of steel are heavy. The weight is increased even more because of the need to make the barrel wall thicker since it cannot be safely heat treated to maximum strength. The heavy barrel is a mere annoyance for hunters and recreational shooters, but it seriously impacts the capability of military systems that must be burdened by the great weight of conventional steel gun barrels. Aircraft must sacrifice load or range to carry the heavy guns using these barrels, reducing the quantity of ammunition the aircraft can carry. The swing weight of large naval guns becomes so great that the train and elevation drives of the guns become immense and slow. The strength needed to resist the high energy propellant loads necessary to achieve ultra-high velocities needed for long range, flat trajectory, high accuracy shooting are practically unattainable because of the great thickness of barrel wall needed, which makes the gun so heavy as to be unmanageable. Moreover, the soft condition of the barrels causes rapid wear of the bore, especially in rapid fire situations where the barrel gets very hot and loses even more of its already low strength. The resultant loss of accuracy of these military weapons make further expenditure of ammunition a total waste.
Corrosion resistance of high carbon steels is notoriously poor. Special coatings and other techniques are available in great profusion to protect the gun barrels from corrosive influences such as salt water, most acids, products of propellant combustion, and many other substances common in the environment. However, most such coatings are most useful if applied frequently, especially immediately after each use of the gun, but it is rarely convenient to do so. Consequently, there is a period following use of the gun before it is cleaned and coated with the protective coating during which rapid corrosion can occur, especially since the combustion products of the propellant, and the projectile fragments remaining in the barrel can create galvanic corrosion. The resultant pitting of the bore then tends to trap additional corrosive materials, further exacerbating the corrosive effects. The effort to find barrel materials that can resist the effects of these corrosive substances has never produced a material that meets the other requirements for a gun barrel.
Vibration and shock of firing large caliber machine guns and artillery tend to be inimical to accuracy. The vibration must be allowed to damp out before the next round is fired or there would be little certainty where the gun will be pointed when the projectile leaves the muzzle. Shock transmitted through the barrel on initiation of the propellant charge may influence the interaction of the projectile in the bore, especially the reflected wave rebounding back from the muzzle. These vibration and shock waves may also interfere with the interaction of the barrel on its mounting structure, and also reduce the life of the gun by fatigue.
Hot plastic deformation of a conventional steel barrel is a serious problem, especially in military guns. At elevated temperatures, the steel barrel is effectively hot forged slightly each time the gun is fired, increasing the internal diameter of the bore slightly and, over time, increasing it enough that the bore, even without erosion, is no longer within bore tolerance. The projectile is loose in such an over-sized bore and has poor accuracy. Moreover, the blow-by of propellant gasses around the projectile in the bore is so great that the projectile does not develop the velocity it needs to attain its specified range, and instead falls short of its intended target.
Long gun barrels present special accuracy problems, especially large caliber guns on the order of 155 mm or larger with cantilevered barrels. Such guns require relatively thick-walled barrels to contain the high propellant gas pressure and provide a large heat sink to prolong the period during which high rate-of-fire can be tolerated before the accuracy deteriorates to the point beyond which further expenditure of ammunition is useless. Such conventional steel thick walled gun barrels are very heavy and have a tendency to droop at the muzzle end when trained at low elevations, especially when the barrel becomes hot and the Young's modulus of the steel drops. These have been intractable problems in the past because of the need for high burst strength and the high density of the only know materials that were proven for use in gun barrels. A composite metal gun barrel that is comparatively light weight, has a high Young's modulus for stiffness, and a high burst strength would be a very welcome development, especially for large caliber guns.
Attempts have been made for years to produce composite gun barrels, always without practical success. The materials used are usually very expensive and labor intensive to build into a barrel. More seriously, however, environmental and service conditions have a destructive effect on composite barrels and no satisfactory solutions to these problems have been developed. The problems include a mismatch of coefficients of thermal expansion between the several elements in the composite barrel, resulting in poor mechanical coupling between those elements and insufficient compressive preload. The attempts to correct these problems are complex and impractical in a production environment. The composite elements tend to be brittle, shock sensitive and vulnerable to attack by common environmental substances such as salt water, as well as acids, hydraulic fluid and other substances common around guns, especially on naval vessels.
Thus, for many years there has been a serious need for a gun barrel, made of tough, high strength materials, that is relatively light weight so that the gun barrel may be made thinner than current barrels and the thin barrel combined with the low density material substantially reduces the weight of the barrel. The high strength and toughness of the barrel materials would permit use of higher energy propellant loads for increased muzzle velocity, range and accuracy. Ideally, the gun barrel would be self damping and immune to the effects of salt water, acids, and the corrosive combustion products of the projectile propellant. Finally, such an ideal gun barrel would have a low coefficient of friction with the projectile materials, a high heat capacity, and low coefficient of thermal expansion to minimize the distorting effects of differential thermal expansion.